# H bridge Calculations

Discussion in 'The Projects Forum' started by Billhc83, Nov 3, 2013.

1. ### Billhc83 Thread Starter New Member

Nov 3, 2013
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Hi I am pretty new to electronics and was looking for a little direction. I am working on building a few H bridges to power a small stepper motor. This is the circuit I am working on. I am trying to use an arduino to drive the inputs.

The motor I am trying to power is 3v and draws 350mA. I am trying to figure out the proper input voltages and resister values for biasing the transistors. I have worked through it a couple of times but cannot get the circuit to work. Does each line of the stepper motor draw 350mA? Any input or suggestions would be greatly appreciated at this point.

2. ### Dodgydave AAC Fanatic!

Jun 22, 2012
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767
It wont work with R9 in the circuit at the 0V supply for the transistors.The motor will draw its current depending on the supply voltage its given, and load resistance, if its a 3V motor, then why are you supplying it with 12V???

3. ### Billhc83 Thread Starter New Member

Nov 3, 2013
2
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Ya the 12v was not accurate. I was trying to use a 6v PS.

4. ### MikeML AAC Fanatic!

Oct 2, 2009
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Why not use FETs? The Arduino port can likely only source/sink <=25ma, which is a bit marginal to drive the bases of T3 and T4. Either use FETs, or Darlington NPNs.

The 1K base resistor in your posted circuit is hopeless. You would need about 35mA of base current.

5. ### tubeguy Well-Known Member

Nov 3, 2012
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You could change the upper 1k base resistors to 100 to 120 ohms for the PNP's , then add 2n2222's to drive the lower NPN's.

6. ### SgtWookie Expert

Jul 17, 2007
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As already insinuated, R9 will cause the lower transistors to not turn on properly. If you ARE going to use R9, you would need to have it on the positive side. Also, not obvious - but you would get better performance if you had Zener diodes rated 2x-4x your supply voltage back to back with each Schottky diode, as otherwise the current will "flywheel" through the diodes for a rather long time when either the high or low side transistors are turned off.